The background description provided herein is for the purpose of generally presenting the context of the disclosure. The work of the inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Manufacturing of semiconductor devices involves deposition of electrically conductive material on substrates such as semiconductor wafers. The conductive material may be deposited by electroplating onto a seed layer of metal such as copper located in vias or trenches.
Electroplating may also be used for through-silicon vias (TSVs), which are connections that pass completely through a semiconductor wafer. Because TSVs are typically large in size and have high aspect ratios, depositing copper can be challenging. CVD deposition of copper for TSVs typically requires complex and relatively expensive precursors. PVD deposition tends to create voids and has limited step coverage. Electroplating is a preferred method for depositing copper for TSVs. However, electroplating also presents challenges due to the large size and high aspect ratio of TSVs.
TSV technology may be used in 3 dimensional (3D) packages and 3D integrated circuits. For example only, a 3D package may include two or more integrated circuits (ICs) that are stacked vertically. The 3D package tends to occupy less space and have shorter communication distances than a corresponding 2D layout.
Wafer Level Packaging (WLP) is an electrical connection technology that, like TSV, employs large features, typically on the scale of micrometers. Examples of WLP structures include redistribution wiring, bumps, and pillars. Electroplating is poised to deliver the next generation of WLP technology.
Damascene processing may be used to form interconnections for integrated circuits (ICs). In a typical Damascene process, a pattern of trenches and vias is etched in a dielectric layer of a substrate. A thin layer of diffusion-barrier film is then deposited onto the dielectric layer. The diffusion barrier film may include a material such as tantalum (Ta), tantalum nitride (TaN), a TaN/Ta bilayer, or other suitable material. A seed layer of copper is deposited on the diffusion-barrier layer using PVD, CVD or another process. Afterwards, the trenches and vias are filled with copper using electroplating. Finally, the surface of the wafer may be planarized to remove excess copper.
The electroplating system may include an electroplating cell with a cathode and an anode that are immersed in electrolyte. One lead of a power supply is connected to the cathode, which includes the copper seed layer. The other lead of the power supply is connected to the anode.
The composition of electrolyte that is used for deposition of copper may vary, but usually includes sulfuric acid, copper sulfate (e.g. CuSO4), chloride ions, and/or a mixture of organic additives. Electrolytes for deposition of other metals will have their own characteristic compositions. Organic additives, such as accelerators, suppressors and/or levelers, may be used to enhance or suppress rates of plating of copper or other metal.
An electric field generated by the applied voltage electrochemically reduces metal ions at the cathode. As a result, metal is plated on the seed layer. The chemical composition of the plating solution is selected to optimize the rates and uniformity of electroplating.
Processes occurring at the anode and cathode are not always compatible. Therefore, the anode and cathode electrolyte may have the same or different chemical compositions. The anode and cathode may be separated by a membrane into different regions. For example only, insoluble particles may be formed at the anode due to flaking of the anode or precipitation of inorganic salts. The membrane may be used to block the insoluble particles, which reduces interference with the metal deposition and contamination of the wafer. The membrane may also be used to confine organic additives to the cathode portion of the plating cell.
The membrane allows the flow of ions (current) between the anode and cathode regions of the plating cell while blocking movement of larger particles and some non-ionic molecules such as organic additives. As a result, the membrane creates different environments in the cathode and the anode regions of the plating cell.
A pump may be used to pump electrolyte to the anode chamber. Periodically, fresh electrolyte and/or deionized water may be introduced to the anolyte flow, which can introduce a transient pressure differential between the electrolyte in the anode chamber and the electrolyte in the remainder of the electroplating cell. This may cause the membrane to deflect upward, which sometimes entraps air next to the membrane. Specifically, the pressure differential can allow air bubbles to trap between the membrane and the support structure. Among other problems, the trapped air will block current from flowing through the region of the membrane occupied by the air and thereby increase the current through other regions of the membrane to introduce plating non-uniformities and significantly shorten the membrane's life. Further, the separation of cathodic and anodic regions produces an electroosmotic effect in which the protons crossing the membrane from the anode chamber to the cathodic portion of the apparatus “drag” water molecules in the same direction thereby depleting the anolyte volume and increasing the volume in the cathode chamber. This effect is known as electroosmotic drag and is undesired as it creates a pressure gradient between the two chambers that can lead to membrane damage and failure.
One approach to prevent damage would be to provide a pressure sensor in the anode chamber to monitor pressure. The sensed pressure value may be fed back in a closed loop control system to control the pressure of the pump. This approach may unfortunately require more expensive pumps that must be precisely controlled with pressure sensors in each anode chamber, which increases cost.